Application Information
GENERAL FEATURES
Mute Function:
The muting function of the LM3886 allows
the user to mute the music going into the amplifier by draw-
ing less than 0.5 mA out of pin 8 of the device. This is
accomplished as shown in the Typical Application Circuit
where the resistor R
M
is chosen with reference to your
negative supply voltage and is used in conjuction with a
switch. The switch (when opened) cuts off the current flow
from pin 8 to V
−
, thus placing the LM3886 into mute mode.
Refer to the Mute Attenuation vs Mute Current curves in the
Typical Performance Characteristics
section for values of
attenuation per current out of pin 8. The resistance R
M
is
calculated by the following equation:
R
M
(|V
EE
| − 2.6V)/I8
where I8
≥
0.5 mA.
Under-Voltage Protection:
Upon system power-up the
under-voltage protection circuitry allows the power supplies
and their corresponding caps to come up close to their full
values before turning on the LM3886 such that no DC output
spikes occur. Upon turn-off, the output of the LM3886 is
brought to ground before the power supplies such that no
transients occur at power-down.
Over-Voltage Protection:
The LM3886 contains overvolt-
age protection circuitry that limits the output current to ap-
proximately 11Apeak while also providing voltage clamping,
though not through internal clamping diodes. The clamping
effect is quite the same, however, the output transistors are
designed to work alternately by sinking large current spikes.
SPiKe Protection:
The LM3886 is protected from instanta-
neous peak-temperature stressing by the power transistor
array. The Safe Operating Area graph in the
Typical Perfor-
mance Characteristics
section shows the area of device
operation where the
SPiKe
Protection Circuitry is not en-
abled. The waveform to the right of the SOA graph exempli-
fies how the dynamic protection will cause waveform distor-
tion when enabled.
Thermal Protection:
The LM3886 has a sophisticated ther-
mal protection scheme to prevent long-term thermal stress
to the device. When the temperature on the die reaches
165˚C, the LM3886 shuts down. It starts operating again
when the die temperature drops to about 155˚C, but if the
temperature again begins to rise, shutdown will occur again
at 165˚C. Therefore the device is allowed to heat up to a
relatively high temperature if the fault condition is temporary,
but a sustained fault will cause the device to cycle in a
Schmitt Trigger fashion between the thermal shutdown tem-
perature limits of 165˚C and 155˚C. This greatly reduces the
stress imposed on the IC by thermal cycling, which in turn
improves its reliability under sustained fault conditions.
Since the die temperature is directly dependent upon the
heat sink, the heat sink should be chosen as discussed in
the
Thermal Considerations
section, such that thermal
shutdown will not be reached during normal operation. Using
the best heat sink possible within the cost and space con-
straints of the system will improve the long-term reliability of
any power semiconductor device.
THERMAL CONSIDERATIONS
Heat Sinking
The choice of a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature at a level such
that the thermal protection circuitry does not operate under
normal circumstances. The heat sink should be chosen to
dissipate the maximum IC power for a given supply voltage
and rated load.
With high-power pulses of longer duration than 100 ms, the
case temperature will heat up drastically without the use of a
heat sink. Therefore the case temperature, as measured at
the center of the package bottom, is entirely dependent on
heat sink design and the mounting of the IC to the heat sink.
For the design of a heat sink for your audio amplifier appli-
cation refer to the
Determining The Correct Heat Sink
section.
Since a semiconductor manufacturer has no control over
which heat sink is used in a particular amplifier design, we
can only inform the system designer of the parameters and
the method needed in the determination of a heat sink. With
this in mind, the system designer must choose his supply
voltages, a rated load, a desired output power level, and
know the ambient temperature surrounding the device.
These parameters are in addition to knowing the maximum
junction temperature and the thermal resistance of the IC,
both of which are provided by National Semiconductor.
As a benefit to the system designer we have provided Maxi-
mum Power Dissipation vs Supply Voltages curves for vari-
ous loads in the
Typical Performance Characteristics
sec-
tion, giving an accurate figure for the maximum thermal
resistance required for a particular amplifier design. This
data was based on
θ
JC
= 1˚C/W and
θ
CS
= 0.2˚C/W. We also
provide a section regarding heat sink determination for any
audio amplifier design where
θ
CS
may be a different value. It
should be noted that the idea behind dissipating the maxi-
mum power within the IC is to provide the device with a low
resistance to convection heat transfer such as a heat sink.
Therefore, it is necessary for the system designer to be
conservative in his heat sink calculations. As a rule, the
lower the thermal resistance of the heat sink the higher the
amount of power that may be dissipated. This is of course
guided by the cost and size requirements of the system.
Convection cooling heat sinks are available commercially,
and their manufacturers should be consulted for ratings.
Proper mounting of the IC is required to minimize the thermal
drop between the package and the heat sink. The heat sink
must also have enough metal under the package to conduct
heat from the center of the package bottom to the fins
without excessive temperature drop.
A thermal grease such as Wakefield type 120 or Thermalloy
Thermacote should be used when mounting the package to
the heat sink. Without this compound, thermal resistance will
be no better than 0.5˚C/W, and probably much worse. With
the compound, thermal resistance will be 0.2˚C/W or less,
assuming under 0.005 inch combined flatness runout for the
package and heat sink. Proper torquing of the mounting
bolts is important and can be determined from heat sink
manufacturer’s specification sheets.
Should it be necessary to isolate V
−
from the heat sink, an
insulating washer is required. Hard washers like beryluum
oxide, anodized aluminum and mica require the use of ther-
mal compound on both faces. Two-mil mica washers are
most common, giving about 0.4˚C/W interface resistance
with the compound.
Silicone-rubber washers are also available. A 0.5˚C/W ther-
mal resistance is claimed without thermal compound. Expe-
rience has shown that these rubber washers deteriorate and
must be replaced should the IC be dismounted.
LM3886
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